Pulsed droplet ejecting system

ABSTRACT

A reservoir supplies liquid through a conduit to a nozzle. The liquid is under small or zero static pressure. Surface tension at the nozzle prevents liquid flow when the system is not actuated. A section of the conduit terminating at the nozzle is designed to be capable of conducting pressure waves in the liquid from end to end of the section without the occurence of significant reflections within the section. An electroacoustic transducer is coupled to the liquid in the reflection-free section. When an electric pulse is applied to the transducer it applies a pressure pulse to the liquid sending a pressure wave to the nozzle where it causes ejection of a droplet. The pressure pulse also sends a pressure wave in the opposite direction. The system has energy absorbing means coupled to the liquid and adapted to absorb substantially all of the energy of the latter wave, thus preventing reflections which could return to the nozzle and interfere with ejection of a subsequent droplet. Two classes of energy absorbing means are described: (a) conduit walls of viscoelastic material which deform under the influence of the pressure wave and absorb energy therefrom, and (b) several forms of acoustic resistance elements within the conduit at the inlet end of the reflection-free section.

United States Patent [191 Arndt [451 Aug. 27, 1974 PULSED DROPLETEJECTING SYSTEM [75] Inventor: John P. Arndt, Cleveland, Ohio [73]Assignee: Gould Inc., Chicago, Ill.

[22] Filed: Feb. 7, 1973 [21] Appl. N0.: 330,360

Primary ExaminerMark O. Budd Attorney, Agent, or Firm-Eber .l. Hyde [57]ABSTRACT A reservoir supplies liquid through a conduit to a nozzle. Theliquid is under small or zero static pressure. Surface tension at thenozzle prevents liquid flow when the system is not actuated. A sectionof the conduit terminating at the nozzle is designed to be capable ofconducting pressure waves in the liquid from end to end of the sectionwithout the occurence of significant reflections within the section. Anelectroacoustic transducer is coupled to the liquid in thereflection-free section. When an electric pulse is applied to thetransducer it applies a pressure pulse to the liquid sending a pressurewave to the nozzle where it causes ejection of a droplet. The pressurepulse also sends a pressure wave in the opposite direction. The systemhas energy absorbing means coupled to the liquid and adapted to absorbsubstantially all of the energy of the latter wave, thus preventingreflections which could return to the nozzle and interfere with ejectionof a subsequent droplet. Two classes of energy absorbing means aredescribed: (a) conduit walls of viscoelastic material which deform underthe influence of the pressure wave and absorb energy therefrom, and (b)several forms of acoustic resistance elements within theconduit at theinlet end of the reflection-free section.

4 Claims, 13 Drawing Figures PATENI AUBZ 71914 SREHMIF 5 IVIIIII 1Pmnmwszmu 7 3.832.579 sum sur 5 PULSED DROPLET EJECTING SYSTEMBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionpertains to a system for ejecting droplets of liquid on command suitablefor use in apparatus such as ink jet printers and facsimile recorders.

2. Description of the Prior Art This invention is an improvement on thesystem described in U.S. Pat. No. 3,683,212, issued to Steven I. Zoltanon Aug. 8, 1972, assigned to the same assignee as the present invention.

A system constructed as described in the Zoltan patent having thedimensions cited by way of example works very well when the pulse rateis less than about one kiloHertz. If the pulsing is continuous and thepulse rate is gradually increased above about one kiloHertz, alternateincreases and decreases in droplet velocity may be observed.

When a burst of pulses equally spaced in time is applied to the system,and the time interval between pulses exceeds about one millisecond, theresulting droplets are ejected with uniform spacing. However, when thetime between pulses is decreased to a fraction of a millisecond, thefirst several droplets which are ejected generally have irregularspacing.

The above described irregularities are undesirable in many applications.An experimental and theoretical investigation has shown that they arecaused by acoustic resonances, reflections, and interference phenomenain the liquid in the system.

OBJECT AND SUMMARY OF THE INVENTION The object of this invention is toprovide a droplet on command system generally similar to the systemdescribed in U.S. Pat. No. 3,683,212 but which is substantially free ofthe irregular performance at high pulse rates observed in systemsconstructed as described in that patent.

According to the invention a reservoir supplies liquid through a conduitto a nozzle. A section of the conduit terminating at the nozzle isadapted to conduct pressure waves in the enclosed liquid without theoccurence of significant reflection within the section. Anelectroacoustic transducer is coupled to the liquid in thereflection-free section of the conduit and is adapted to apply apressure pulse to the liquid whereby a first pressure wave travels inthe liquid to the nozzle and causes ejection of a droplet therefrom, anda second pressure wave travels in the liquid toward the inlet end of thereflection-free section. An energy absorbing means is coupled to theliquid in the conduit and is adapted to absorb substantially all of theenergy of the second pressure wave.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of thepresent invention, together with other and further objects thereof,reference is had to the following description taken in connection withthe accompanying drawings, and its scope will be pointed out in theappended claims.

In the drawings:

FIG. 1 shows a system according to the invention partly in section andpartly schematic;

FIG. 2 shows a test set up for selecting certain system parameters;

FIG. 3 shows graphs obtained with the set up of FIG. 2;

FIGS. 4 to 11 inclusive show modifications of the system of FIG. 1;

FIG. 12 is an exploded view of a system according to the invention whichdiffers substantially in mechanical detail from the system shown in FIG.1; and

FIG. 13 is a conventional sectional view along lines 13--l3 of FIG. 12.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, areservoir shown schematically at 1 contains ink or other liquid 2. Aconduit indicated generally by reference character 4 communicates withliquid 2 in the reservoir and is filled with the liquid. Conduit 4terminates in a nozzle 10 which also is filled with liquid 2. Droplets13 of the liquid may be ejected on command through orifice ll of thenozzle as will be explained in later paragraphs.

Conduit 4 comprises a section 5 having an inlet end 7. Section 5 isformed of a material such as glass which provides a smooth internalsurface and relatively stiff walls. The internal cross-sectional area issubstantially constant along the length of section 5. At dashed line 8the cros-sectional area begins a gradual reduction, forming nozzle 10having exit orifice 11. Therefore section 5 may be regarded as having anoutlet end at 8, and this end is terminated by nozzle 10.

Conduit 4 also comprises a liquid supply section 14 formed ofviscoelastic material such as a plasticized polyvinyl chloride.

The internal diameter of supply section 14 is smaller than the internaldiameter of section 5. Section 14 is expanded at one end and forced overthe outside of section 5 at inlet end 7 thereof. Supply section 14 maycontinue to reservoir 1 where it may terminate below the surface ofliquid 2, or it may be coupled to an additional section 16 leading fromreservoir 1.

A tubular electroacoustic transducer 17 surrounds conduit section 5 andis secured thereto in stress transmitting engagement by epoxy cement 19.Preferably transducer 17 comprises a piezoelectric lead zirconateleadtitanate ceramic tube 20, having electrodes 22, 23 on the cylindricalsurfaces, and is radially polarized. A metal foil strip 25 is insertedto provide electrical contact with electrode 22 prior to introduction ofcement l9.

Terminal wire 26 is wrapped around conduit section 5 in contact withfoil strip 25 and is secured in electrical contact therewith byconductive epoxy 28. Terminal wire 29 is wrapped around electrode 23 andsecured in electrical contact therewith by conductive epoxy 31.

Due to the well known piezoelectric effect, the inside diameter oftransducer 17 decreases almost instantaneously when a voltage ofsuitable polarity is connected between terminal wires 26 and 29. Thisdiameter decrease forces decrease in diameter of the portionof conduitmember 5 which is surrounded by transducer 17. Liquid 2 within thatportion of section 5 must therefore either be compressed, or experiencesome displacement. As the voltage between terminals 26 and 29 is reducedto zero, transducer 17 and conduit member 5 return to their originaldimensions, again causing pressure change in liquid 2, or displacementthereof.

Thus transducer 17 is coupled to the liquid in conduit section 5.

Reservoir l is maintained at an elevation which applies little or nopressure to the liquid 2 in nozzle 10. A slight negative pressure, onthe order of two to three centimeters of head seems to be advantageous.Under quiescent conditions, the surface tension of the liquid in orificell prevents flow of liquid 2 in either direction.

When it is desired to have a droplet ejected from nozzle 10, a voltagepulse of the polarity which causes contraction of the transducer isapplied between terminals 26 and 29. The transducer contracts inresponse to the pulse causing slight decrease in the internal volume ofconduit member 5. This momentarily compresses that portion of liquid 2which is within transducer 17 and causes pressure waves to travel in theliquid toward outlet 8 and nozzle and also toward inlet 7 and reservoir1.

Conduit section 5, surrounded over part of its length by transducer 17,may be regarded as an acoustic transmission line. By virtue of therelatively stiff walls, and the uniform cross-sectional area of theenclosed liquid along the length of the section, it conducts pressurewaves in liquid 2 substantially without the occurrence of reflectionswithin the section.

The pressure wave which travels in the liquid toward outlet end 8 ofsection 5 causes ejection of a droplet from nozzle 10.

When supply section 14 of conduit 4 is formed of appropriate materialand is suitably porportioned as hereinafter described, thecharacteristic acoustic impedance looking into the liquid in section 14from inlet end 7 of section 5 is approximately matched to thecharacteristic acoustic impedance of section 5. Thus the pressure wavewhich travels in the liquid from transducer 17 toward inlet end 7 ofconduit section 5, passes into the liquid within section 14 withoutdeleterious reflection. The wave therefore continues in the liquid insection 14 toward reservoir 1. As the wave progresses it causes elasticdeformation of the viscoelastic material of supply section 14,progressively along the length thereof. Since the material isviscoelastic, part of the energy transferred from the liquid to thematerial to cause deformation is converted to heat and is not returnedto the liquid as potentialor kinetic energy as the wave passes. Thus, asthe wave progresses toward reservoir 1, the energy of the wave isprogressively absorbed by conduit section 14.

When the attenuated wave reaches the reservoir end of conduit section 14it encounters an impedance discontinuity with consequent reflection backtoward inlet end 7 of conduit section 5. As the reflected wave travelsthrough section 14 toward inlet 7 of section 5 it is attenuated byabsorption in the viscoelastic material in the manner above described.Conduit supply section 14 is made long enough so that the energy of thereflected wave, when it reaches nozzle 10, is too low to havesubstantial influence on the ejection of a new droplet when a newvoltage pulse is applied to terminals 26 and 29. Thus the viscoelasticmaterial of supply section 14 may be regarded as energy absorbing meanscoupled to the liquid in conduit 4 which absorbs substantially all ofthe energy of the wave which travels from transducer 17 toward inlet end7 of conduit section 5.

As the electric drive pulse decays to zero, transducer 17 and conduitsection 5 return to their original dimensions. After a droplet has beenejected, the liquid in nozzle 10 withdraws from the end thereof leavingan empty space which is then refilled by liquid from the conduit underthe urging of capillary forces in the nozzle. Following refill of thenozzle, quiescent conditions prevail until another electric drive pulseis applied to transducer 17. When a new pulse is applied, the abovedescribed process repeats. Thus droplets may be ejected on command, eachcommand being given by applying an electric pulse to transducer 17.

The pulse shape requirement is not critical. It has been foundadvantageous to have rise time less than two microseconds dwell time offive to flfty microseconds, and fall time greater than two microseconds.Good results also have been obtained using a cosine squared pulse shapewith period of ten to one hundred microseconds.

Many electric circuit arrangements can be devised for generating andapplying suitable electric drive pulses. For examples of such circuits,reference may be made to US. Pat. No. 3,683,212 to Zoltan.

In order for the system to operate as described it is necessary to havea suitable inter-relationship between the properties of the materialforming supply conduit section 14, the dimensions of section 14, theinside diameter of conduit section 5, and properties of liquid 2. If aproper relationship is not established, a pressure wave traveling in theliquid from transducer 17 will be at least partially reflected when itreaches inlet end 7 of section 5. When that reflected wave reachesnozzle 10 it may cause ejection of an additional, undesired droplet, orit may interfere with the desired ejection of a new droplet whichhappens to be timed to occur as the reflection reaches the nozzle. Whenthe reflected wave reaches the nozzle it will be at least partiallyreflected back toward inlet 7, and upon arrival at inlet 7 this newlyreflected wave will be reflected just as the original wave fromtransducer 17 was reflected. In severe cases of incorrect matching ofsupply section 14 to section 5 a large number of reflections may thustake place before the energy decreases enough so as not to interferewith ejection of another droplet initiated by a new command pulse. Thus,the stronger the reflections, the longer the time interval before a newdroplet can be ejected without disturbance from the reflecting waves.

Selection of suitable viscoelastic material and dimensions for conduitsection 14 to prevent deleterious reflections at inlet end 7 of conduitmember 5 may be accomplished by testing, as hereinafter described, aseries of sample sections constructed of various materials and with arange of dimensions for each material. For use in such testing, theassembly of FIG. 1 is provided with an additional transducer 32 securedto conduit section 5 close to nozzle 10. Transducer 32 may be identicalto transducer 17 except that preferably it is made much shorter. Foilstrip 34 is inserted and terminal wires 35, 37 are secured by conductingepoxy 38,40 just as in the case of strip 25, terminal wires 26,29 andepoxy 28,31 associated with transducer 17.

The tests may be performed using the dual transducer assembly of FIG. 1in a test set-up as shown in FIG. 2. In FIG. 2 the liquid supply section14 under test is expanded at one end and forced over the inlet end 7 ofconduit section 5 as in FIG. 1. A hypodermic syringe 41 fitted with ablunt needle 43 selected for a snug fit in section 14' serves as areservoir. Syringe 41 is loaded with liquid 2 of the kind that will beused with the droplet ejecting system, and the liquid is forced throughthe conduit to eject a stream from nozzle until all air is swept out ofthe system. Thereafter, during the test, no pressure is required butcare must be exercised to prevent drawing liquid back out of nozzle 10into conduit section 5.

A variable frequency sine wave oscillator 44 is connected by coaxialcable 46 to terminals 26,29. Oscillator 44 preferably has a continuousfrequency range from below 1,000Hz to 50KHz. A substantially constantoutput voltage is desirable. A level of about two volts is satisfactory.

A motor drive mechanism 47 is mechanically coupled to the frequencycontrol dial 49 of oscillator 44 to sweep the oscillator slowly over theentire frequency range. A potentiometer 50, supplied with current fromdc source 52 also is coupled to motor drive 47. The output ofpotentiometer 50 goes to the x-axis terminals 52 of an XY plotter 53.Thus, as oscillator 44 is swept over its frequency range, the pen 55 ofXY plotter 53 is driven across chart 56.

The swept frequency voltage from oscillator 44 causes transducer 17 andthe portion of conduit section 5 surrounded by transducer 17 toalternately increase and decrease in diameter in synchronism with theoscillator voltage. These dimensional variations cause correspondingpressure variations in the liquid 2. The amplitude of the preseurevariations is too small to cause ejection of droplets from nozzle 10.However, the pressure variations within transducer 32 adjacent to nozzle10 stress the transducer sufficiently toproduce measurable AC voltagebetween terminals 35,37, corresponding to the pressure variations. Thisvoltage may be in the range of one to ten millivolts when conduitsection 14' is properly matched to section 5, and much higher when thereis a serious mismatch.

The pressure pickup signal which is developed between terminals 35 and37 is applied to an electronic AC voltmeter 58 via coaxial cable 59.Meter 58 has output terminals 62 between which appears a DC signalproportional to meter deflection. Terminals 62 connect to Y-axisterminals 64 of XY plotter 53. Thus, as oscillator 44 is swept over itsfrequency range, the X Y plotter draws a graph of pressure behind nozzle10 .vs frequency. A pressure calibration of the system is not requiredbut it is desirable to have a rough calibration of the X or frequencyaxis.

Preferably meter 58 is a tuned voltmeter with tuning dial 65 coupled tooscillator dial 49 in a manner which insures'accurate tracking. The useof a tuned meter reduces difficulties that may otherwise be encounteredwith pickup of noise and stray signals. Instruments are commerciallyavailable which combine in one unit the functions of oscillator 44,tracking tuned voltmeter 58, sweep drive 47, potentiometer 50, and DCsupply 52. One such instrument is the model 302A Wave Analyzer equippedwith Model 297A Sweep Drive, manufactured by the Hewlett PackardCompany. One of these instruments was used in the tests hereinafterdescribed.

In continuing the description of the test procedure reference will bemade to a particular series of tests that resulted in the selection ofmaterial'and dimensions for conduit section 14 that later produced goodresults in actual pulsed droplet ejection. Referring to FIG. 1,approximate specifications and dimensions were as follows:

Conduit section 5 lime glass length 2.5 cm inside diameter 0.05] cm wallthickness 0.0l cm Transducer 17 Lead Zirconate-Lead Titanate Ceramiclength L25 cm inside diameter 0.076 cm wall thickness 0.025 cmTransducer 32 Lead Zirconate-Lead Titanate Ceramic length 0.16 cm insidediameter 0.076 cm wall thickness 0.025 cm Nozzle l0 orifice diameter0.007 cm Liquid 2 distilled water FIG. 3 is a copy of XY plots forseveral sample conduit members 14'. Curve 67 was obtained with a conduitsection 14 made of soft vinyl material. The inside diameter was 0.063 cmand the outside diameter was 0.16 cm. The pressure peak at about 15' kHzoccurred because conduit section 14' was too large in inside diameter,and was too soft, providing very low acoustic impedance, thus allowingalmost complete reflection at inlet end 7 of conduit section 5. Nearlycomplete refiection also took place at nozzle 10 because of the veryhigh acoustic impedance presented by the nozzle, representing nearly ablocked condition. Thus the 15 kHz peak may be regarded as a quarterwave resonance in conduit section 5. The peak at about 40 kHz may beregarded as a three quarter wave resonance. The lack of exact 3 to 1correspondence probably is due to the imf pedance of conduit member 14'not being zero and the impedance of the nozzle not being infinite.

Curve 68 in FIG. 3 was obtained with a conduit section 14' of muchstiffer material. The inside diameter was 0.063 cm and the outsidediameter was 0.18 cm. The reduced height of the peaks at 15 kHz and 40kHz indicates that a significant part of the energy of the wavetraveling in the liquid from transducer 17 toward conduit section 14'continued into the liquid in section 14 as desired.

Curve 70 in FIG. 3 was obtained with a conduit section 14' made of thesame material and having the same outside diameter involved in curve 68but the inside diameter was 0.041 cm. This curve indicates substantiallyreflection-free transmission from conduit section 5 to section 14 andrepresents a satisfactory selection of material and dimensions. Theslight hump in the curve at about 23 kHz suggests that possibly aslightly larger inside diameter would be preferable,-but pulsedoperating tests of a system employing this conduit section producedsubstantially uniform drop ejection at rates up to 10 kHz, which is anorder of magnitude improvement over the results obtained'with a dropleton command system constructed as desribed in U.S. Pat. No. 3,683,212. A

Curve 71 in FIG. 3 was obtained with a conduit section 14' made of thesame material and having the same outside diameter involved in curves 68and 70 but having inside diameter of 0.025 cm. The low response at-ISkI-Iz and the pronounced hump at about 25kHz indicate that the insidediameter was too small.

An additional series of tests provides a useful guide to determining theminimum length for conduit section 14'. After selecting suitablematerial and diameters for section 14 as illustrated by the examplereferring to FIG. 3, the selected sample is cut to one-half length and anew curve is run and compared with the curve for the original length.The section just tested is again cut to half length and a new curve isrun and again compared with the original curve. This process is repeateduntil a new curve is obtained which is significantly different from theoriginal curve. At this point it may be assumed that the length is tooshort and the preceding length should be considered to be approximatelythe minimum length.

The conduit section 14' which resulted in curves 68, 70,71 in FIG. 3were made of a plastisol prepared as follows:

Ingredients and Source Parts by Weight Resin vinyl chloride homopolymeridentified as Geon 121 powder, supplied by B. F. Goodrich Chemical Co.,Cleveland, Ohio 70 Plasticizer diactyl phthalate,

identified as Good-Rite GP261 Plasticizer, supplied by B. F.

Goodrich Chemical Co., Cleveland,

Stabilizer identified as 6-V-6-A Stabilizer, supplied by Ferro ChemicalDivision, Ferro Corp., Bedford, Ohio I The plasticizer and stabilizerwere added to the resin powder in a beaker and hand stirred for overthirty minutes. This formed a very stiff mixture which then was placedin a bell jar and evacuated and held for 24 hours.

The conduit section was moulded by forcing the thick mixture to fill thespace around a smooth wire tensioned and centered within a glass tube,and then curing at a temperature of 160C for about one to three minutes.The wire then was stretched beyond its elastic limit to reduce thediameter, and withdrawn. The viscoelastic tube then was withdrawn fromthe surrounding glass tube which formed part of the mold.

Satisfactory pulsed droplet ejections over a wide pulse frequency rangehas been obtained with a system having the parameters tabulated aboveand provided with conduit section 14' formulated as described and havinginside diameter of 0.041 cm, and length of 10 cm. This was the conduitmember resulting in curve 70 of FIG. 3.

Similar results have been obtained employing an extruded section ofplasticized polyvinyl chloride tubing supplied by the Norton Company ofAkron, Ohio, under the Trade Mark TYGON. The outside diameter was 0.178cm and the inside diameter was 0.041 cm, and the length was 10 cm. Theparticular TYGON composition was identified as formulation S54-HL.

Preferably conduit section is formed of glass, but metal, and plasticsections have been used successfully. Preferably nozzle if formedintegrally with conduit section 5 in a manner to provide a smoothcontour as illustrated in FIG. 1. However, an abrupt transition from therelatively large diameter of conduit section 5 to the small diameter oforifice 11 does not mitigate against satisfactory operation of thisinvention.

The use of viscoelastic material to form part of conduit 4 is not theonly way to provide energy absorbing means coupled to the liquid in theconduit. Another way that has produced satisfactory results is toinstall a suitable acoustic resistance element at the inlet end 7 ofconduit section 5 through which the liquid 2 flows, as shown in FIGS. 4to 11.

Referring to FIG. 4, the construction may be generally similar to theconstruction shown in FIG. 1. Conduit section 5, preferably of glass, isterminated at the outlet end at 8 by nozzle 10. Transducer 17 is securedto section 5 by epoxy cement l9, and terminal wires 26,29 are attachedas in FIG. 1. A pressure measuring transducer is not shown in FIG. 4 butif desired one may be provided by using a longer conduit section 5 andattaching a transducer 32 as in FIG. 1.

One form of acoustic resistance unit that has given satisfactory resultshas been provided by pressing a short bundle of glass fibers 73 into theend extension 74 of conduit section 5 as shown in FIGS. 4 and 5. Theresistance element terminates at dashed line 7' which marks theeffective inlet end of conduit section 5.

Liquid 2 is supplied from a reservoir, now shown, through supply conduitsection 76 which may be made of any convenient material. Preferably itis formed of soft plastic and has inside diameter equal to or largerthan the inside diameter of conduit section 5. If desired, conduitextension 74 may be attached directly to the reservoir, eliminatingsection 76.

Another form of acoustic resistance unit that has given good results hasbeen provided by filling conduit extension 74 with minute glass beads 77and then fusing them together and to the inner wall of extension 74 asshown in FIGS. 6,7.

Still another successful resistance unit has been provided by pressing acylinder 79 of porous plastic into conduit extension 74 as shown inFIGS. 8,9. The particular material that was employed was cut from aPOROSYN tip for a TIP-WIK pen sold by the Eversharp Pen Company ofJanesville, Wisconsin.

The flow resistance R of the acoustic resistance elements of FIGS. 4 to9 should be approximately equal to the characteristic acoustic impedanceZo of the liquid filled conduit section 5. The characteristic impedanceZ0 is given approximately by:

Z0 l/S) V B P where S 1r a cross sectional area of the liquid columnenclosed by conduit section 5 a inside radius of conduit section 5 Bbulk modulus of liquid 2 p density of liquid 2 Suitable dimensions anddegree of packing of the glass fibers 73 of FIGS. 4,5; suitabledimensions and degree of fusing of the beads 77 of FIGS. 6,7; orsuitable material and length for the porous cylinder 79 of FIGS. 8,9 toobtain the desired value of R may be determined by experiment. Theexperimental resistance element to be tested should be vacuumimpregnated with the liquid to be used, and then such liquid should beforced through the unit under measured pressure P taking care to avoidintroducing any air. The quantity Q of liquid which flows in a measuredtime t should be measured. The resistance then is calculated from:

As an alternative, experimental resistance units may be installed in adouble transducer assembly similar to that shown in FIG. 1, but suppliedwith liquid as in FIG.

4, and then tested in the set-up of FIG. 2. Resistance which is too lowwill result in curves resembling curves 67 or 68 in FIG. 3. When theresistance is too high the curve should resemble curve 71. The correctresistance should produce a curve resembling curve 70 of FIG. 3.

FIGS. 10 and 11 show how an acoustic resistance may be provided by anannular slit at the inlet end 7 of conduit section 5. In this case,conduit section 5 and the nozzle, not shown, preferably are made ofmetal or plastic. Small indentations 82 are formed in conduit extension74. A solid cylinder 80 of metal or plastic is pressed into extension 74and held in place by indentations 82. The acoustic resistance R is givenapproximately by R 6p.L/ t 1ra where [.L viscosity of liquid 2 L axiallength of cylinder 80 t= clearance between cylinder 80 and conduitextension 74 a inside radius of conduit extension 74 The dimensions tand L should be selected to make R approximately equal to thecharacteristic impedance Z of conduit section 5.

FIGS. 4 to 11 show various ways in which energy absorbing means in theform of acoustic resistance elements may be coupled to the liquid in theconduit to absorb substantially all of the energy of a pressure wavetraveling in the liquid from transducer 17 toward the inlet end 7 ofconduit section 5. In each case the inlet end 7 may be located inside oftransducer 17 as illustrated in FIGS. 4 and 8 or may be external of thetrans ducer as illustrated in FIGS. 6 and 10.

In any of the constructions of FIG. 1 and FIGS. 4 to 11 it isadvantageous to enclose the transducer 17 in a jacket of yieldablematerial such as rubber or plasticized vinyl. The assembly may thus besecured in the apparatus in which it is used, by clamping to the jacket.With such an arrangement there is little danger of applying breakingstresses to the assembly, and the clamp does not interfere significantlywith the pulsing changes in diameter of the assembly as droplets areejected. The jacket may be in the form of a section of tubingdimensioned to fit tightly over the transducer as shown at 78 in FIG. 4,or it may be cast around the assembly. In the latter case it may extendover the end of the transducer embedding also the wrapped-aroundportions of terminal wires 26, 29 and the exposed portion of conduitsection or'74. Another advantage of such a jacket is that it providesdamping of mechanical resonances of the assembly which might otherwisecause deleterious effects.

It is not necessary to employ a cylindrical piezoelectric transducersurrounding the conduit as shown in FIGS. 1 and 4 to 11. Othertransduction principles may be used, for example, electrostriction andmagnetostriction. Further, other geometric configurations and methods ofcoupling the transducer to the liquid may be employed. For example, inFIGS. 12 and 13 the transducer is a piezoelectric disc which is coupledto the liquid by direct contact.

In FIGS. 12, 13, piezoelectric disc 83, preferably of leadzirconate-lead titanate ceramic, has electrodes 85,86 to which terminalwires 88, 89 are attached by solder or conductive epoxy 91,92.

Piezoelectric disc 83 is clamped between metal or plastic cover plates94,95 by O-rings 97,98 which fit into grooves 100,101 in the coverplates. Terminal wires 88,89 extend through openings 103,104 in thecover plates.

Also clamped between covers 94,95 is a sheet 106 of I cut-out 109 at oneend thereof. Opening 110 through cover 94 communicates with cut-out 109at the other end. Thus there is formed a conduit comprising tubularmember 107, cut-out 109 enclosed by covers 94, 95, opening 110, and anannular space formed by cut-out 1 12, the rim of piezoelectric disc 83,O-rings 97,98 and cover plates 94,95. The conduit is terminated at oneend by sapphire watch jewel 113 which serves as a droplet ejectingnozzle. The other end of the conduit, i.e., the open end of tubularmember 107, may be immersed in liquid in a reservoir, not shown, or maybe coupled to liquid in a reservoir by an additional conduit member suchas a flexible tube. The entire conduit and the opening 115 in nozzle 113are filled with the liquid.

To facilitate further description, the section of the above describedconduit extending'from dashed line 118 to the face of watch jewel 113 atdashed line 116 will be identified as conduit section 118-116. Line 118marks the inlet end and line 116 marks the outlet end. The locationselected for line 118 is not critical but preferably it is considered tobe near or at conduit member 107. The internal cross sectional areas ofthe various components of conduit section 118-116 are selected so thatpressure waves in the liquid may travel from end-to-end of the sectionwithout the occurrence of significant reflection within the section.

The polarization of piezoelectric disc 83 is in the thickness direction.Thus, when a voltage of suitable polarity is connected between terminals88 and 89, the diameter of the disc increases. When the voltage isreduced to zero, the disc returns to its original diameter.

The rim of piezoelectric disc 83 forms part of conduit section 118-116and is in direct contact with the liquid. O-rings 97,98 which also formpart of the conduit prevent the liquid from contacting electrodes 83,85.Thus, when a voltage pulse with polarity that causes increase ofdiameter is applied to transducer 83 the liquid surrounding thetransducer is momentarily compressed. This causes a pressure wave totravel through the liquid in conduit section 118-116 to the outlet end116 thereof and eject a droplet from nozzle 113. It also causes apressure wave to travel through the liquid toward inlet end 118. As thelatter wave progresses from the rim of transducer disc 83 it causeselastic deformation of the viscoelastic material of sheet 106progressively along the length of conduit section 118-116, withconsequent absorption of wave energy as described in connection withFIG. 1. After the wave passes inlet end 118 it at some point encountersan impedance discontinuity and therefore it is at least partiallyreflected. As the reflected wave progresses toward nozzle 113 itexperiences further attentuation due to energy absorption in theviscoelastic walls of the conduit. The conduit section 118-116 is madesufficiently long so that the reflected wave energy reaching nozzle 1 13is too weak to interfere with ejection of subsequently initiateddroplets.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is aimed,therefore, in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A pulsed droplet ejecting system comprising:

a reservoir;

liquid contained in said reservoir;

a conduit communicating with said liquid in said reservoir and filledwith said liquid, said conduit comprising a first section having aninlet end and an outlet end;

a nozzle terminating said outlet end of said section and filled withsaid liquid; and

an electroacoustic transducer coupled to said liquid in said section;

said section, with said transducer coupled to the liquid therein, beingdimensioned to conduct pressure waves in said liquid between said endssubstantially free of internal reflections and being dimensionedrelative to the properties of said liquid to have a given characteristicacoustic impedance;

said transducer being adapted to apply a pressure pulse to said liquidwhereby a first pressure wave travels in said liquid in 'said firstsection to said nozzle and causes ejection of a droplet therefrom, andwhereby a second pressure wave travels in said liquid in said firstsection toward said inlet end of said section;

said conduit comprising a second section which is attached to the inletend of said first section and is comprised of viscoelastic material andis dimensioned relative to the properties of said liquid and to theproperties of said viscoelastic material to have characteristic acousticimpedance substantially matching said characteristic acoustic impedanceof said first section so that the said second pressure wave travels insaid liquid from said first section into said second section withoutdeleterious reflection at said inlet end of said first section and whichis dimensioned relative to properties of said viscoelastic material sothat said second wave is substantially fully absorbed by saidviscoelastic material.

2. A pulsed droplet ejecting system as described in claim 1 in whichsaid transducer is a piezoelectric transducer.

3. A pulsed droplet ejecting system as described in claim 2 in whichsaid first conduit section is cylindrical and in which saidpiezoelectric transducer surrounds said section and is in stresstransmitting engagement therewith.

4. A pulsed droplet ejecting system as described in claim 3 in whichsaid second section is a viscoelastic tube having inside diametersmaller than the inside diameter, of the inlet end of said firstsection, except that said second section is enlarged where it isattached to said inlet end with the inner surface of said enlargedportion engaging the outer surface of said inlet end.

1. A pulsed droplet ejecting system comprising: a reservoir; liquidcontained in said reservoir; a conduit communicating with said liquid insaid reservoir and filled with said liquid, said conduit comprising afirst section having an inlet end and an outlet end; a nozzleterminating said outlet end of said section and filled with said liquid;and an electroacoustic transducer coupled to said liquid in saidsection; said section, with said transducer coupled to the liquidtherein, being dimensioned to conduct pressure waves in said liquidbetween said ends substantially free of internal reflections and beingdimensioned relative to the properties of said liquid to have a givencharacteristic acoustic impedance; said transducer being adapted toapply a pressure pulse to said liquid whereby a first pressure wavetravels in said liquid in said first section to said nozzle and causesejection of a droplet therefrom, and whereby a second pressure wavetravels in said liquid in said first section toward said inlet end ofsaid section; said conduit comprising a second section which is attachedto the inlet end of said first section and is comprised of viscoelasticmaterial and is dimensioned relative to the properties of said liquidand to the properties of said viscoelastic material to havecharacteristic acoustic impedance substantially matching saidcharacteristic acoustic impedance of said first section so that the saidsecond pressure wave travels in said liquid from said first section intosaid second section without deleterious reflection at said inlet end ofsaid first section and which is dimensioned relative to properties ofsaid viscoelastic material so that said second wave is substantiallyfully absorbed by said viscoelastic material.
 2. A pulsed dropletejecting system as described in claim 1 in which said transducer is apiezoelectric transducer.
 3. A pulsed droplet ejecting system asdescribed in claim 2 in which said first conduit section is cylindricaland in which said piezoelectric transducer surrounds said section and isin stress transmitting engagement therewith.
 4. A pulsed dropletejecting system as described in claim 3 in which said second section isa viscoelastic tube having inside diameter smaller than the insidediameter of the inlet end of said first section, except that said secondsection is enlarged where it is attached to said inlet end with theinner surface of said enlarged portion engaging the outer surface ofsaid inlet end.